“Plasma enhanced chemical vapor deposition” (or PECVD) is a known technique used to form films on various substrates. For example, Felts et al. U.S. Pat. No. 5,224,441 describes an apparatus for rapid plasma deposition. In the plasma enhanced chemical vapor deposition of silicon oxide, a gas stream including components such as a volatilized organosilicon compound, oxygen, and an inert gas such as helium or argon, is sent into an enclosed chamber at reduced pressure and a glow discharge plasma is established from the gas stream or its components. A silicon oxide layer is deposited upon the substrate when it is positioned near the plasma. In such a system, the pressure is typically reduced from atmospheric pressure by a vacuum pumping system. Electrode surfaces are in electrical communication with the gases introduced into the system such that an electrical discharge or plasma is formed. The purpose of this discharge is to excite moieties in the system and cause them to be deposited onto the workpiece or substrate to be coated.
The use of the “hollow cathode effect” is known from the applicant's own patent application W02006/019565 (Boardman et al.), in which the internal surface of tubes and pipes are modified by a treatment process in which the workpiece itself forms the deposition chamber. Treatment is effected within the workpiece by applying a biasing voltage between an electrode within the workpiece, or just at the exterior of the workpiece, and the workpiece itself whilst passing a treatment gas through the workpiece and maintaining the interior of the workpiece at a reduced pressure. The treatment gas contains the desired element to be deposited or implanted and the pressure is low enough to establish and maintain the “hollow cathode effect” in which the electron mean free path is slightly less than the diameter of the workpiece, thus causing electron oscillation and implantation or deposition of the desired element below or onto the surface of the component itself.
One problem in such systems is that the electrodes can become contaminated with an insulating layer of the material intended for the component. The growth of deposits on the electrodes will result in a shift of the voltage/current characteristics of the system over time and the required operating voltage will increase for a given current as contamination of the electrode progresses. These changes cause a drift in the quality of the dielectric coating produced on the substrate, and require periodic cleaning or replacement of the electrodes.
Furthermore, due to the high impedance presented to the plasma by a contaminated electrode, power will be wasted and excess heat generated. It is desirable to have a process free of drift with a minimum of waste heat generated. A similar problem occurs when the counter electrode must be placed at the exterior of the workpiece electrode. Many times this must be done when the diameter of the tube being coated is small, or if pre-activation of the plasma is needed to provide a uniform coating down the tube. In this case there is a resistance between the high density, hollow cathode plasma in the workpiece (usually biased as the cathode, as described later) and the counter electrode (usually biased as the anode), due to the decay of the plasma as it flows from the cathode to the anode. This problem will of course only become worse if the anode becomes coated with resistive material.
A further problem resides in the fact that one must heat the electrode to thermionic emission temperatures in order to release electrons, and in some arrangements the electrode is so hot that one needs to employ a separate and expensive water cooling apparatus simply to prevent the heat from the electrode adversely affecting the surrounding structure. An example of such an arrangement is shown in Countrywood et al. U.S. Pat. No. 6,110,540, which discusses a counter electrode employed in a deposition process. In FIGS. 3 and 4, for example, a substantial electrode is surrounded by a cooling system which cools the material surrounding the electrode. This patent also discloses the possible use of a coil of refractory metal wire as a substitute for the electrode of FIG. 4 but makes no further comment in connection therewith.
Countrywood et al. requires the addition of a separate magnetic field generating system in order to constrain the generated plasma within the desired region of the anode structure. Such systems introduce additional cost and complexity to an otherwise already complex system and consume additional electrical energy.
Countrywood et al. also requires an AC signal such that the counter electrode alternately acts as an anode and then a cathode. In the hollow tube electrode configurations described in Countrywood et al., a negative (cathode) bias on the counter electrode is required for a portion of the waveform, to generate the intense low impedance plasma (essentially, a hollow cathode plasma). Countrywood et al. does describe a DC anode, but this requires a separate direct current power supply to power the anode. An advantage of the present invention is that a positive bias on the counter electrode is not required, nor is a separate power supply required for DC or DC pulse processes, as in Countrywood et al.
The above-mentioned patent also discloses the use of a gas purge system in which the electrode is protected from the treatment gas by a shielding gas which is passed over the electrode and passes out of a chamber in which the electrode is located via a small hole provided therein. The shielding gas is provided at a higher pressure than the treatment gas, and thus acts to prevent any treatment gas entering the electrode chamber and being deposited thereon. The gas supply associated with the electrode can maintain the gas pressure around the electrode greater than in other areas of the evacuated chamber. These gases form a relatively high-density plasma associated with the electrode, which acts as an extension of the metallic electrode surface and lowers the impedance of the electrode. This electrode system is commonly known as a gas purged electrode. The benefit of using a gas purged electrode is that it provides a constant, low impedance electrical contact with the process plasma. Since the impedance is constant, the process does not drift; since impedance is low, the whole process operates at a lower voltage and less power is wasted. The greater gas pressures around the gas purged electrode are continually replenished by the gas supply, and the gas pressure differential between the area around the gas purged electrode and other areas of the evacuated chamber prevents reactive gases or other components from the main gas supply from approaching the gas purged counter electrode. The gas employed is generally an inert gas, such as helium, neon, or argon, or a mixture such as helium/neon or neon/argon. In reactive sputtering processes it can be oxygen, nitrogen or other reactive gases. Each of these gases may be used in the apparatus and method of the present invention.